Present day tuners do not work sufficiently well for processing digital signals received using various transmission systems. Both the RF and IF characteristics of the receivers need improvement since a digital signal occupies the entire spectrum of a channel, and delay and frequency response errors have a more severe effect on digital signals than on analog signals. Moreover, since the transmitted power of digital signals is considerably less than the transmitted power for analog transmissions, acceptable reception of the desired signal can be more difficult in the presence of strong adjacent channel signals. Further, the signal conditions in the fringe area of over-the-air transmissions are also a problem. For fringe areas, a few tenths of a dB loss of signal-to-noise ratio (SNR) or an increase of intermodulation distortion, can make the signal recovery difficult, if not impossible.
Conventional automatic gain control AGC systems in TV receivers typically are responsive to the level of the demodulated video signal. After comparison of the demodulated video signal with a reference, error voltages are generated to control the gain of the IF amplifier and RF stage of the tuner. To obtain a good SNR over a wide range of input levels, it is a common practice to delay the application of AGC to the tuner until relatively high signal levels are encountered. This works well in the absence of strong adjacent signals. However, if the signal level is low in the presence of strong adjacent signals, cross modulation with the strong adjacent signals could occur in the mixer and the bit error rate (BER) of the demodulated television information would increase.
Typically, there is a single AGC control signal for the IF and a single AGC control signal for the RF, which may or may not be delayed. Additionally, the AGC control signal for the RF typically is derived from the AGC control signal for the IF. Although the relative amplitudes of the AGC control signals can be made different for the various stages by the use of dividers, the overall ratios between the various stages remains constant for various signal levels. More particularly, the AGC control signal for the RF is not individually adjustable according to the characteristics of the input signal, especially for optimizing the tuner characteristics with respect to adjacent channel signals and SNR.
Finally, the ultimate criterion for the quality of digital reception is the BER, which is effected by SNR and intermodulation distortion. Currently, there is much research being conducted to develop improved devices to minimize such difficulties in the tuner. However, even if such new devices are successful, the limitations of the mixer will still be troublesome. Thus, it is desirable that the quality of the signal coupled to various stages of a tuner be optimized independently of the devices used in the various tuner stages.